The present invention relates in general to coatings for protecting metal articles and more particularly to cathodic protection by synthetic elastomer and/or natural rubber coatings for pipelines intended for inground implantation.
Various protective pipeline coatings are well known in the art. In addition, federal regulations require that all major pipelines be cathodically protected in order to markedly decrease failures due to corrosion. Cathodic protection is defined as reduction or elimination of corrosion by making the metal a cathode by means of an impressed direct current or attachment to a sacrificial anode, usually magnesium, aluminum, or zinc.
While turning the entire structure into a cathode will eliminate corrosion in general, breaks and imperfections in the pipeline coating, do present special problems. Initially, such breaks or "holidays" are protected by the negative charge, vet with time and in part due to moisture and minerals in the soil, it is precisely this negative charge which accelerates undercutting and disbondment of the coating system in the areas of holidays.
For these reasons, the pipeline coating art has heretofore devoted considerable attention to the task of providing cathodic disbondment resistance to the various protective coating systems employed.
While not intended as a complete survey of the art of resisting cathodic disbondment, the following patents are believed to be fairly illustrative.
U.S. Pat. No. 3,876,606 issued to Kehr relates to thermosetting epoxy resin powders for coating metal articles which are said to exhibit exceedingly good resistance to disbondment. In accordance with the invention, the epoxy resin composition comprises a homogenous blend of a polyglycidyl ether of a polyhydric phenol having a softening point of 70.degree.-120.degree. C., a specified dihydrazide hardening agent, and, as the essential novelty, at least 15% by volume of barium sulfate and/or calcium carbonate, up to one-third of which may be substituted by mica powder.
According to the patentee, U.S. Pat. Nos. 3,102,043 and 3,578,615, cited therein, appear to indicate that certain fillers for powdered epoxy resin, especially mica powder, improve resistance to disbondment.
U.S. Pat. No. 4,009,224 issued to Warnken also relates to epoxy resin powders used to provide protective coatings, especially for petroleum pipelines, the essence of the invention apparently being incorporating a copolymer of vinyl acetate and ethylene.
U.S. Pat. No. 4,027,059 issued to Koons relates to specified asphalt-based compositions for coating and for patching coatings on pipelines consisting of a precursor composition comprising asphalt, at least one polyhydroxy polymer, at least one polyhydroxy compound, at least one polyamine, at least one solvent and optionally, at least one filler. This precursor composition, where combined with at least one polyisocyanate is said to form a composition especially useful for coating and for patch coating and which is said to provide resistance to cathodic disbonding upon cure.
U.S. Pat. No. 4,133,352 also issued to Koons relates to primer coatings for asphalt-based pipe coatings which will improve the properties of the asphalt-based coatings, particularly their hot line adhesion and cathodic disbonding resistance. The primer coating comprises a solution of chlorinated rubber and aromatic petroleum pitch in a solvent.
U S. Pat. No. 4,192,697 describes a method for protecting pipes which is said to exhibit no loss of bond when tested in accordance with ASTM G8-69T, which method comprises wrapping helically over the pipe a hot fabric strip impregnated with a hot molten adhesive, e.g. asphaltic bitumen, coal tar or rubber containing asphaltic bitumen and coal tar, and then overwrapping with a strip of plasticized polyvinyl chloride sheet, under tension, to form a sheathing.
U.S. Pat. No. 4,213,486 issued to Samour et al (and assigned to the assignee of the instant application) describes a pipewrap system having improved cathodic disbondment properties wherein the pipe is first coated with an epoxy and thereafter a flexible tape outerwrap is applied, the tape preferably being a hot melt or pressure-sensitive adhesive carried on a polyolefinic layer.
U.S. Pat. No. 4,455,204 discloses an adhesive coating for pipes having improved resistance to cathodic disbondment comprising an adhesive component and a specified hydrazine derivative.
U.S. Pat. No. 4,523,141 and a division thereof, 4,589,275, both issued to Thomas et al and assigned to the instant assignee, is directed to test procedures for determining whether the outer barrier layer in a pipewrap system is in fact free from pinholes or other small discontinuities which can induce cathodic disbondment, so that any necessary repairs can be made before the pipe is laid in the ground. In acccordance with the claims of the parent case, U.S. Pat. No. 4,523,141, the method for detecting pinholes in a pipecoating having a thermal insulating layer and an outer barrier layer by measuring electrical conductivity or resistivity along the face of the barrier layer, is improved if an electroconductive intermediate layer is interposed between and bonded to, the insulating and barrier layers. In accordance with the divisional case, U.S. Pat. No. 4,589,275, the intermediate layer for facilitating testing can be electroconductive, smooth surfaced or of a color contrasting to the barrier layer.
As was previously alluded to, the aforementioned patents are not intended to constitute a comprehensive state of the art pertaining to prevention of cathodic disbondment. They are, however, considered to be fairly illustrative of the state of the art known to applicant.
Particularly efficacious systems for protecting metal pipes and the like are those comprising a rubber-based primer coating and an adhesive tape outerwrap. While these rubber-based pipewraps provide exceptionally fine protection against corrosion and other degradative environmental forces, the cathodic protection voltage applied to the inground pipeline will nevertheless cause at least some cathodic disbondment to occur. Accordingly, there is a need for some means for inhibiting or preventing cathodic disbondment in pipewrap systems.
The task of this invention may accordingly be said to be to provide protection against cathodic disbondment to a pipewrap system comprising a synthetic elastomer and/or natural rubber primer coating applied to the surface of the pipe and an outer rubber-based adhesive tape wrapped over the primer coating.